A DP-QPSK (Dual Polarization Quadrature Phase Shift Keying) modulation system which multiplexes an X-polarization QPSK signal and a Y-polarization QPSK signal that are obtained by quadrature-phase modulating an X-polarization light and a Y-polarization light with independent information, respectively, is promising as the modulation system for large-capacity signal transmission. In this DP-QPSK modulation system, since the bit rate is 40 Gbit/s when the symbol rate is 10 GSymbol/s and the bit rate is 100 Gbit/s when the symbol rate is 25 GSymbol/s, the frequency utilization efficiency can be increased.
In the DP-QPSK modulation system receiver (DP-QPSK receiver), a DP-QPSK signal is separated into an X-polarization QPSK signal and a Y-polarization QPSK signal by a polarization beam splitter (PBS). Furthermore, using a 90-degree hybrid for X polarization and a 90-degree hybrid for Y polarization, the separated X-polarization QPSK signal and Y-polarization QPSK signal are mixed with a local oscillation light (LO light), respectively. Thus, the phase modulation signal (QPSK signal light) of each polarized wave is converted into an intensity modulation signal, respectively. Then, for the signal light in each polarized wave, the components (I component and Q component) corresponding to a real part and an imaginary part of an electric-field amplitude are independently extracted, when the electric-field amplitude is expressed by a complex number. Note that, the 90-degree hybrid is generally known as a circuit, wherein an input signal light and a local oscillation light are branched into two, respectively, and a 90-degree phase difference is given between the two-branched local oscillation lights as a relative lightwave phase difference, and thereafter one of the two-branched signal lights is mixed with one of the two-branched local oscillation lights and the other one of the two-branched signal lights is mixed with the other one of the two-branched local oscillation lights.
Non-Patent Documents 1 to 3 disclose a 90-degree hybrid which is formed by combining a coupler and a PBS on one planar lightwave circuit (PLC). Moreover, Non-Patent Document 4 discloses a 1×4 DQPSK demodulator including a delay circuit and a 90-degree hybrid formed on a PLC. This 90-degree hybrid circuit is configured as a part of a Mach Zehnder interferometer (MZI), wherein an MMI coupler is used in an input/output section of the MZI.